Widespread fears have been raised over medical tourism with foreign nationals arriving in Britain solely to use the NHS for emergency surgery or to have their babies.

The costs of such care should be recovered by the hospital treating them.

However little is known about the burden on GP practices with rules stating that GPs have a contractual duty to provide free emergency treatment and immediate necessary treatment for up to 14 days to any person within their practice area.

Practices can, at their discretion, accept overseas visitors for inclusion in their patient list if the person is staying more than three months, but cannot discriminate on grounds of race.

GPs can refuse to take on patients from outside their catchment area and can ask for proof of address for these reasons but they must ask all potential patients or none to avoid allegations of discrimination.

The Pulse survey of 229 GPs, found 15 per cent of practices were breaking these rules by asking for addresses only some of the time.

NHs guidance says GPs can only refuse a patient if their practice is full or in exceptional circumstances such as the patient having a history of violence towards doctors.

Foreign nationals do not pay for GP consultations but do have to pay for prescriptions.

An ongoing Government review is examining NHS entitlement in primary care and whether hospital-style charges could be extended.

Dr Paul Roblin, chairman of Berkshire, Buckinghamshire and Oxfordshire Local Medical Committee, which represents GPs in the area, said the lack of clarity is a major source of frustration for GPs.

"The Department of Health has abrogated its responsibility. It has failed for decades to provide a rule book that is understandable to all.

"You could argue whether it is fair for visitors and migrants to have entitlement to NHS treatment when that is not reciprocated.

"GPs are expected to practice with limited healthcare funding and if we are using that money for treating visitors, the taxpayer loses out.

"It may be more appropriate to have a system where we say that if you can’t provide proof of residence, you have to pay for treatment."

The British Medical Association has advised doctors not to make checks on residency as there is no obligation on them to do so and it leaves them open to allegations of discrimination.

Dr Richard Vautrey, deputy chairman of the General Practice Committee at the BMA, said: "If GPs ask for evidence of immigration status or residency from one patient, but not every other patient, they could be deemed to be applying discrimination and that’s something that they should not be doing – so they either need to do it for every patient or do it for none.

"I think it’s better to accept that it’s a role for other bodies in the health service to assess whether someone is eligible for NHS care or not – not for GPs."

A Department of Health spokesman said: “The NHS is not a global health service and we are committed to having a system of entitlement that is fair and affordable to the taxpayer.

“We have recently concluded a wholesale review of charges for primary and secondary care to address concerns about the rules on access to free treatment and to cut down abuse to ensure that those who should pay do so. We expect to make further announcements soon."